The principle of a digital optical display comprising a seven-segment display for ciphers may be derived from FIGS. 13 and 14 of publication DT-AS No. 19 19 798 with the respective portions of its specification. A light source is mounted on one side of a code plate moved by a watch movement in angular increments. At each angular step a certain code pattern defined by holes in the plate is passed between the light source and the light inlets of fibre optical light guides, the other ends of said light guides being deformed to define the bars of the seven-segment display and being spacedly arranged in a corresponding fashion.
The input signal to be displayed basically is the angular position of the movement spring housing representative of the time period expired since the rewinding of the watch movement. This input signal is already digital due to the watch escapement system.
If it is desired to optically display analog input signals, too, and make use of such a digital display additional problems will occur since the stroke or the angle through which the measuring member has moved and which is representative of the analog value to be displayed must be definitely allocated to a certain number of stroke or angle increments of the code plate. It is evident that suitably the light inlets of the light guides must be aligned with that code of the code plate whose digital value is most approximate to the analog value to be displayed. This involves that somewhere in the system the analog value must be digitalized so that the interpolation makes sense.
A solution for this problem is described and illustrated in DT-OS No. 23 49 764. Therein the analog measured value, viz. the load of a scale is transduced into a proportional rotation of the code plate whose angular position, thus, is still an analog value. The code plate further is provided with a uniform optical index corresponding to the digial increments and optically scanned. By means of a servo loop the light inlets are moved relative to the code plate, the servo signal being derived from said optical index.
Although the last-metioned system is operational it has the drawbacks of being extremely complex and cumbersome. Even if instead of a proportional automatic control a two point control is provided which necessitates less effort, the code on the other hand must be made one and a half times larger than the light inlets so that as a result, the space necessary for the system is still increased. Finally, a source for auxiliary energy for the servo mechanism is indispensable.
It has been tried already to provide the code plate with a preferably magnetic indexing system as described and illustrated for example in DT-OS No. 23 33 195. In the latter publication, however, there is not an optical code plate but one having conductive and non-conductive, respectively, area, this code plate being electrically scanned and serving likewise the digital display of weights. Although there are provisions made to eliminate the reaction due to the friction of the scanning electrodes there is nevertheless no operative display reliable within the entire range of the measurements. The reason is that the indexing system necessarily acts for each increment with substantially the same moving force upon the code plate, said force reacting upon the measuring system and disbalancing the position thereof. If the balanced position as in a scale is defined by balance between the weight load and a removing deflection proportional spring bias, the new balance position will result just in a more or less approximated alignment of code and scanning elements depending upon the beginning or the end of an increment. In result one would have to use larger code area than normally necessary for the digitalizing.